ABB RELION 620 Series Technical Manual page 886

Section 13
Requirements for measurement transformers
13.1.1.2
880
The accuracy classes 5P and 10P are both suitable for non-directional overcurrent
protection. The 5P class provides a better accuracy. This should be noted also if there are
accuracy requirements for the metering functions (current metering, power metering, and
so on) of the protection relay.
The CT accuracy primary limit current describes the highest fault current magnitude at
which the CT fulfils the specified accuracy. Beyond this level, the secondary current of the
CT is distorted and it might have severe effects on the performance of the protection relay.
In practise, the actual accuracy limit factor (F
(F ) and is proportional to the ratio of the rated CT burden and the actual CT burden.
n
The actual accuracy limit factor is calculated using the formula:
S + S
in n
F ≈ F ×
a n
S + S
in
A071141 V1 EN
F the accuracy limit factor with the nominal external burden S
n
S the internal secondary burden of the CT
in
S the actual external burden
Non-directional overcurrent protection
The current transformer selection
Non-directional overcurrent protection does not set high requirements on the accuracy
class or on the actual accuracy limit factor (F
to select a CT with F of at least 20.
a
The nominal primary current I
dynamic strength of the current measuring input of the protection relay is not exceeded.
This is always fulfilled when
I > I / 100,
1n kmax
I is the highest fault current.
kmax
The saturation of the CT protects the measuring circuit and the current input of the
protection relay. For that reason, in practice, even a few times smaller nominal primary
current can be used than given by the formula.
) differs from the rated accuracy limit factor
a
) of the CTs. It is, however, recommended
a
should be chosen in such a way that the thermal and
1n
1MAC504801-IB E
n
620 series ANSI
Technical Manual